\defineprocessor [Upper] [command=\Word]
\defineprocessor [Red]   [color=red]

\starttext

anything\setregisterentry[index][keys=Anything, entries=Anything]

something\setregisterentry[index][keys=Something, entries=Something]

\page

Anything\setregisterentry[index][keys=Red->,entries=Upper->anything]

Something\setregisterentry[index][keys=Red->,entries=Upper->something]

\page

\placeindex

\stoptext